Introduction: This study examined the effects of acute caffeine intake on muscular electrical activity during strength, power, and endurance performance tests at different times of day in bench press and back squat exercises.
Methods: Thirteen resistance-trained men participated in a triple-blind, cross-over, randomized controlled trial with four conditions: (a) morning with caffeine, (b) morning with placebo, (c) evening with caffeine, and (d) evening with placebo. Trials were conducted at 9:00 AM and 6:00 PM, with participants consuming caffeine or placebo (3 mg/kg) 60 min prior. Muscular strength/power was tested at 25%, 50%, 75%, and 90% one-repetition maximum (1RM) for bench press and back squat, while muscular endurance was assessed at 65% 1RM through a set-to-failure. Surface electromyography (EMG) measured muscle electrical activity.
Results: In muscular strength/power tests, caffeine increased mean velocity (Vmean) and power output (Wmean) in the back squat at 75% (P = 0.012-0.001, g = 3.84-2.71) and 90%1RM (P = 0.043-0.009, g = 2.77-2.46) in both morning and evening trials. At 25%1RM, caffeine counteracts morning performance decline in bench press (10-11%, P = 0.001, g = 2.62-1.68) and back squat (8-11%, P = 0.010-0.003, g = 2.22-1.64). In muscular endurance tests, caffeine increased Vmean and Wmean in bench press in the morning (11-12%, P = 0.003-0.005, g = 2.55-1.89) and back squat in both morning and evening trials (6-9%, P = 0.001-0.027, g = 2.79-1.73). EMG activity remained unchanged in all conditions.
Conclusions: Acute caffeine intake enhances muscular strength/power at moderate-to-high loads (75%- 90%1RM) and endurance performance (65%1RM) in the back squat while counteracting morning declines at light-load (25%1RM) for both exercises without altering muscle electrical activity.
Keywords: Caffeine; Dietary supplements; Resistance training; Sports performance; Surface electromyography.
© 2025. The Author(s).